128 PEKIDOTITE. 



tose aspect. Tlie section shows a reticulated network of opacite, with interspaces havin^^ 

 a fibrous border and a granular centre of serpentine. The section further contains some 

 enstatite altered to serpentine, magnetite, and a little picotite, or chroniite, and hematite. 

 This is regarded as an altered olivine-enstatite rock. Further examination of these Alpine 

 serpentines showed that they were either derived from rocks of this character, or else 

 from oliviue-auoite-eustatite rocks.* 



Variety. — Lherzolite. 



Lalic Lhcrs, France. 



The famous lherzolite occurring about Lake Lherz, and at various localities between 

 that lake and Vicdessos and Sem, in the department of Ariege, in the Pyreneean region of 

 Southern France, has been described by Professor T. G. Bonney as a crystalline aggregate 

 of olivine, enstatite, and diallage (diopside), with some picotite ; the texture varying fi-om 

 a finely to a coarsely granular. Color on the fresh fracture, a dark greenish-gray or olive- 

 green. The rock on close inspection shows specks of emerald-green diallage, waxy look- 

 ing, dull-green serpentine, resinous, pale-brown enstatite, and minute grains of picotite, 

 inclosed in the predominant dull-colored, or glassy, olivine mass. 



The sections are grayish to water-clear aggregates of olivine, enstatite, and diallage, 

 holding picotite. The section is traversed by a network of fissures, and is thus coarse or 

 fine-granular in different portions. The olivine is in rounded, water-clear, more or less 

 irregular grains, and is the predominant mineral, forming, according to Zirkel and Bonney 

 two-thirds of the whole mass of the rock. Tlie enstatite is clear, colorless, and sometimes 

 shows a slight, silky texture. The diallage, like the enstatite, is in irregular fragments, 

 sometimes clear and transparent, and at others shows a faint tinge of green. Both it and 

 the enstatite are often feebly dichroic, varying from colorless to various pale shades of 

 green. Sometimes the diallage varies simply in the depth of the green tint. These min- 

 erals are not to be certainly distinguished one from the other, except by their optical 

 characters. 



The picotite occurs in coffee-brown, irregular masses and grains, the latter often 

 grouped together in little masses, scattered along from the ends of some larger mass. 

 The color is sometimes a yellowish-green, and Professor Bonney describes some as being 

 of a deep olive-green. I should regard the picotite as being the first formed mineral, 

 instead of the last, as he regards it. In some portions of the sections serpentine has been 

 formed along the fissures, showing fibrous polarization, the fibres sometimes lying parallel, 

 sometimes perpendicular to the walls. Near these serpentine veins the olivine is dark- 

 ened along its fissures, apparently from the separation of magnetite or chroniite in a fine 

 powder. In some cases these black grains are united into irregular, branching, spiney 

 masses. Masses of these black aggregations are seen arranged in the centre of the vein- 

 lets of the serpentine, like islets in a stream. Professor Bonney, in his sections, was able 

 to trace the alteration of the olivine to serpentine, one of his sections showing a network 

 of serpentine veins surrounding and penetrating the other minerals. In the sections 

 before me, the olivine is in some cases changed to a pale-greenish serpentine, holding 

 minute aggregations of the ferruginous grains. These serpentine masses are generally iso- 

 tropic, although showing in a few points the fibrous aggregate polarization of serpentine. 



* Bomiey, Geol. Mag., 18S0 (2), vii. 538-542. 



